1. Field of the Invention
The present invention relates to an illumination system for a projection device, and more particularly, to an illumination system for a projection device adapted to generate a visible light image and an infrared light image.
2. Descriptions of the Related Art
Human eyes can only see the visible light with wavelength ranging from approximately 400 nm to 700 nm. However, at night or when the visible light becomes faint, light (e.g., moonlight or starlight) irradiated from the objects in the natural environment has wavelength mainly ranging from approximately 700 nm to 1,000 nm, so human eyes cannot see the objects in such environment. In particular, it is critical for military operations to see clearly the targets at night. Therefore, in military and civil aviation applications, simulation at night with a training simulator is one of the primary training programs for aviation training. A common night vision goggle (NVG) can convert the light with wavelength between 650 nm and 880 nm (mainly an infrared light) into a visible light with wavelength between 400 nm and 700 nm. In most cases, human eyes preferably receive visible light with wavelength of about 550 nm that is converted by the NVG.
In reference to FIGS. 1A and 1B, a conventional night vision projector 1 can generate a visible light image and an infrared light image. In reference to FIG. 1A, during the simulation of a visible light image, by turning on a light source 11 and turning off a light source 12, a light ray 11a generated by the light source 11 passes through a visible light filter 112, is then reflected by reflectors 131, 133 to a light processing element 14 to be processed into a visible light image, and is finally projected outward via a lens 15. In reference to FIG. 1B, during the simulation of a night vision image, by turning off the light source 11 and turning on the light source 12, a light ray 12a generated by the light source 12 is filtered by an infrared light filter 122 into an infrared light, is then reflected by reflectors 132, 134 to the light processing element 14 to be processed into an infrared light image, and is finally projected outward, also via the lens 15, to present an infrared light image. At this point, the user observes the infrared light image by using the NVG to simulate conditions at night. However, both the light ray 11a and the light ray 12a emitted by the light sources 11, 12 have to be filtered through the visible light filter 112 and the infrared light filter 122 and then be reflected by the reflectors 131, 132, 133, 134 multiple times respectively to form the visible light image and the infrared light image respectively; therefore, the light ray 11a and the light ray 12a are continuously attenuated during the filtering process and the multiple reflections described above, which affects the luminance of the generated images. Furthermore, the visible light filter 112 and the infrared light filter 122 will absorb a great deal of light energy that is filtered out and thus, generates a great deal of heat energy, resulting in many problems. On the other hand, the night vision projector 1 of this kind usually has disadvantages, such as a bulky volume due to the numerous elements therein.
FIG. 2 illustrates another conventional night vision simulation system 2. A tracking element 24 tracks the viewing direction of a night vision goggle 21, an image generator 23 controls a display 22 to generate a corresponding red light image for output to the night vision goggle 21 according to the viewing direction of the night vision goggle 21. In this way, although the user can experience the image that is saw when a night vision goggle is used, this simulator requires complicated and precise parts and fails to allow multiple persons to see the same infrared light image at the same time, which reduces their the feeling of presence of drilling at night. On the other hand, conventional night vision simulation system 2 cannot provide a visible light image; and when a visible light image is to be provided for use in daylight, another auxiliary apparatus is required.
In view of this, a need exists in the art to provide an illumination system for a projection device, which can provide both a common visible light image and an infrared light image for simulating conditions at night without the need of complicated parts and apparatuses and also, can increase the luminance and the light utilization efficiency and reduce the generation of heat energy.